1. Field of the Invention
The present invention relates to a projection optical system, a production method thereof, and a projection exposure apparatus using it. More particularly, the invention concerns a projection optical system used for transferring a predetermined mask pattern onto a substrate by use of a light source of an ultraviolet region suitable for semiconductor fabrication, a production method thereof, and a projection exposure apparatus using it.
2. Related Background Art
An example of the conventional projection exposure apparatus for semiconductor fabrication is one having the structure as illustrated in FIG. 16A and FIG. 16B.
Specifically, the projection exposure apparatus 800 illustrated in FIG. 16A is constructed in such structure that rays from a light source 501 such as a mercury-arc lamp or the like are collected by an ellipsoidal mirror 502 and that thereafter they are converted into a bundle of parallel rays by a collimator lens 503. Then this parallel beam travels through a fly""s eye lens 504, which is an aggregate of optical elements 504a of a square section as illustrated in FIG. 16B, to form a plurality of light source images on the exit side thereof. An aperture stop 505 having a circular aperture is disposed at the position of the light source images. Beams from the plurality of light source images are condensed by a condenser lens 506 to uniformly illuminate a reticle R as an object to be illuminated, in superimposed fashion.
A pattern on the reticle R kept under uniform illumination by the illumination optical system in this way is projected onto a wafer W coated with a resist, by a projection optical system 507 consisting of a plurality of lenses. This wafer W is mounted on a wafer stage WS, which is arranged to move two-dimensionally, and the projection exposure apparatus 800 of FIG. 16A is designed to perform exposure by the so-called step-and-repeat method in which the wafer stage is successively moved two-dimensionally in order to implement exposure in a next shot area after completion of exposure in one shot area on the wafer.
In recent years proposals have been made about such scanning exposure methods that a rectangular or arcuate beam was radiated onto the reticle R and that the reticle R and wafer W located in conjugate with each other with respect to the projection optical system 507 were moved in a fixed direction whereby the pattern of the reticle R could be transferred in high throughput onto the wafer W.
In the projection exposure apparatus in either of the above methods, it is desirable that optical members used in their optical systems have high transmittance at the wavelength of the light source used. The reason is as follows: the optical systems of the projection exposure apparatus are constructed of a combination of many optical members; even if optical loss per lens is little the total transmittance will be decreased greatly when the optical loss is added up by the number of optical members used. If an optical member inferior in transmittance were used, it would absorb the exposure light to increase the temperature of the optical member itself and thus cause heterogeneity of refractive index and, in turn, local thermal expansion of the optical member would deform polished surfaces. This would degrade the optical performance.
On the other hand, the projection optical systems are required to have high homogeneity of refractive index of the optical members in order to achieve a finer and sharper projection exposure pattern. The reason is that variations in refractive index will cause a lead and a lag of light and this will greatly affect the imaging performance of the projection optical system.
Thus silica glass or calcium fluoride crystals high in transmittance in the ultraviolet region and excellent in homogeneity are generally used as materials for the optical members used in the optical systems of the projection exposure apparatus in the ultraviolet region (not more than the wavelength of 400 nm).
Proposals of decreasing the wavelength of the light source have been made recently in order to transfer a finer mask pattern image onto the wafer surface, that is, in order to enhance the resolution. For example, decrease of wavelength into a shorter range is under way from the g-line (436 nm) and the i-line (365 nm), which have been used heretofore, to KrF (248 nm) and ArF (193 nm) excimer lasers.
In the projection exposure using such shorter-wavelength excimer lasers, since the purpose is to obtain the finer mask pattern, the materials used are those with higher performance as to the homogeneity of transmittance and refractive index.
With use of such materials having the high homogeneity of transmittance and refractive index, however, there were cases wherein desired resolution was not obtained after the optical system was assembled in combination of plural materials.
An object of the present invention is, therefore, to provide a projection optical system having high imaging performance, a production method thereof, and a projection exposure apparatus capable of achieving high resolution.
The inventors have conducted intensive and extensive research in order to accomplish the above object and first found out that birefringence of the materials of the optical members greatly affected the imaging performance of the projection optical system and the resolution of the projection exposure apparatus. Then the inventors discovered that the imaging performance close to designed performance of the projection optical system and the resolution close to designed performance of the projection exposure apparatus were attained if the magnitude of birefringence, i.e., birefringence values (absolute values) of the materials of the optical members were not more than 2 nm/cm and if distribution of birefringence was symmetric with respect to the center in the optical members, and disclosed it in Japanese Patent Application Laid-Open No. 8-107060.
With increase in demands for much higher resolution of the projection exposure apparatus, however, there were cases wherein satisfactory imaging performance of the projection optical system and satisfactory resolution of the projection exposure apparatus were not attained even with employment of the above conventional design concept if light of the shorter wavelength was used as a light source or if an optical member having a large diameter and a large thickness was used.
Thus the inventors have conducted further research and, as a result, discovered that the cause of failing to obtain the projection optical system and projection exposure apparatus of desired optical performance even with use of the optical members having good homogeneity of transmittance and refractive index was that distribution states of birefringence of the optical members differed among the optical members, the different birefringence distributions were added up in the overall optical system where the projection optical system was constructed in combination of a plurality of optical members, and this resulted in disturbing the wavefront of the light in the overall optical system, thereby greatly affecting the imaging performance of the projection optical system and the resolution of the projection exposure apparatus.
Describing the above in more detail, the conventional ways of evaluating the birefringence of optical members were nothing but arguments about whether the magnitude (absolute values) was high or low, and there was no concept of the above distribution of birefringence in the optical members at all, either. For example, for measuring the birefringence of a silica glass member, it was common recognition to those skilled in the art to measure the birefringence at several points near 95% of the diameter of the member and employ a maximum as a birefringence value in the member. However, the inventors precisely measured the distribution of birefringence of silica glass members and found that the actual distribution of birefringence was nonuniform.
Therefore, the inventors found out that influence of birefringence in each member was not able to be evaluated sufficiently by simply managing the maximum of birefringence values at several points in the member even if the silica glass member had high uniformity of refractive index and, particularly, that it was very difficult to obtain an optical system of desired performance in combination of plural members. The reason why such nonuniform distribution of birefringence values is formed in the silica glass member is conceivably that the nonuniform distribution of birefringence values is formed in the member during cooling of the silica glass member because of temperature distribution during synthesis, nonuniform distribution of impurities, or nonuniform distribution of structural defects of SiO2.
Since the evaluation of birefringence in the overall optical system constructed of a plurality of optical members was not able to be expressed simply by only the magnitude of birefringence in the individual optical members as discussed above, the inventors precisely investigated how the nonuniform distribution of birefringence values in the optical members affected the optical system. As a result, the inventors first discovered that, with attention being focused on directions of the fast axis as to the nonuniform distribution of birefringence values, the birefringence values were added up to negatively affect the performance of the optical system when the optical system was constructed of optical members having their respective distributions of birefringence values with the same direction of the fast axis, and that negative effects due to the birefringence of the individual members canceled out in the overall optical system where optical members having different directions of the fast axis were combined conversely, and thus accomplished the present invention.
Namely, a projection optical system of the present invention is a projection optical system having at least two silica glass optical members, wherein the optical members are combined with each other so as to satisfy such a placement condition that a signed birefringence characteristic value of the entire projection optical system is between xe2x88x920.5 and +0.5 nm/cm both inclusive, said signed birefringence characteristic value being calculated in such a manner that a birefringence value is measured at each of points in a plane normal to the optical axis with a center at an intersection with the optical axis in each optical member, a distribution of signed birefringence values in each optical member is obtained based on a plurality of birefringence values and directions of the fast axis thereof, and the signed birefringence characteristic value of the entire optical system is calculated based on the distributions of signed birefringence values.
When the optical members are combined so as to satisfy the above placement condition based on the signed birefringence values, the nonuniform distributions of birefringence values in the individual optical members can be quantitatively evaluated with attention on the directions of the fast axis and the optical system can be assembled while quantitatively estimating the signed birefringence characteristic value of the entire optical system from the signed birefringence values of the optical members so as to cancel the distributions of birefringence in the optical members with each other, thereby obtaining the projection optical system with good imaging performance.
A projection exposure apparatus of the present invention is a projection exposure apparatus comprising an exposure light source, a reticle in which a pattern original image is formed, an illumination optical system for illuminating the reticle with light emitted from the exposure light source, a projection optical system for projecting a pattern image outputted from the reticle, onto a photosensitive substrate, and an alignment system for achieving alignment of the photosensitive substrate with the reticle, wherein the projection optical system is the projection optical system of the present invention described above.
The provision of the projection optical system of the present invention permits the projection exposure apparatus of the present invention to attain excellent resolution.
Further, a production method of the projection optical system according to the present invention is a production method of a projection optical system having at least two silica glass optical members, the production method comprising a step of measuring a birefringence value at each of points in a plane normal to the optical axis with a center at an intersection with the optical axis in each optical member and obtaining a distribution of signed birefringence values in the plane normal to the optical axis, based on a plurality of birefringence values and directions of the fast axis thereof, a step of calculating a signed birefringence characteristic value of the entire projection optical system, based on the distributions of signed birefringence values of the respective optical members, and a step of combining the optical members with each other so as to satisfy such a placement condition that the signed birefringence characteristic value of the entire projection optical system is between xe2x88x920.5 and +0.5 nm/cm both inclusive.
The concept of xe2x80x9csigned birefringence valuexe2x80x9d in the present invention will be described below.
The signed birefringence value means a birefringence value provided with a sign in consideration of the direction of the fast axis defined in the index ellipsoid in the measurement of birefringence values of an optical member.
More specifically, in the plane normal to the optical axis with the center at the intersection between the optical member and the optical axis, an area under circular irradiation of light is defined as a nearly circular, effective section, the plus (or minus) sign is assigned to a birefringence value measured when the direction of the fast axis in a small area at a birefringence measuring point on this effective section is parallel to a radial direction from the center at the intersecting point between the optical member and the optical axis, and the minus (or plus) sign is assigned when perpendicular.
The above sign assigning method to the birefringence values can also be applied to cases wherein a plurality of beams are radiated into the plane normal to the optical axis with the center at the intersecting point between the optical member and the optical axis. In such cases, the plus (or minus) sign is assigned to a birefringence value measured when the direction of the fast axis in a small area at a birefringence measuring point on an effective section of each of the areas under illumination with the plurality of beams is parallel to a radial direction from the center at the intersection between the optical member and the optical axis, and the minus (or plus) sign is assigned when perpendicular.
Further, the above sign assigning method to the birefringence values can also be applied to cases wherein the beams have the shape other than the circular cross section in the plane normal to the optical axis with the center at the intersection between the optical member and the optical axis, for example, to cases of beams of a ring cross section or an elliptic cross section. In these cases, the plus (or minus) sign is assigned to a birefringence value measured when the direction of the fast axis in a small area at a birefringence measuring point on an effective section of each of the areas under illumination with the plurality of beams is parallel to a radial direction from the center at the intersection between the optical member and the optical axis, and the minus (or plus) sign is assigned when perpendicular.
The following will describe the cases wherein the plus sign is assigned to a birefringence value measured when the direction of the fast axis in the small area at the birefringence measuring point on the effective area under irradiation with the beam is parallel to a radial direction from the center at the intersection between the optical member and the optical axis and the minus sign is assigned when perpendicular.
The signed birefringence value will be described below in further detail with reference to FIG. 1A, FIG. 1B, FIG. 2A, FIG. 2B, FIG. 3A, and FIG. 3B.
FIG. 1A is a schematic diagram to show directions of the fast axis at birefringence measuring points P11, P12, P13, and P14 the distance r1, r2, r3, or r4, respectively, apart from the center O1 on the effective section of the optical member L1. In this figure the birefringence measuring points P11 to P14 are positioned on a straight line Q1 extending radially through the center O1 for convenience""s sake of description. In the figure, the size of a small area indicated by a circle at each measuring point corresponds to an optical path difference at each measuring point. Directions of segments W11, W12, W13, and W14 in these small areas indicate the directions of the fast axis. Since all the directions of the fast axis at the measuring points P11 to P14 are parallel to the direction of the straight line Q1, i.e., to the radial direction, all the birefringence values at the measuring points P11 to P14 are expressed with the plus sign. When these signed birefringence values A11, A12, A13, A14 at the measuring points P11 to P14 illustrated in FIG. 1A, obtained as described above, are plotted against the distance in the radial direction, the distribution obtained is, for example, the profile as illustrated in FIG. 1B.
FIG. 2A is a schematic diagram to show directions of the fast axis at birefringence measuring points P21, P22, P23, P24 the distance r1, r2, r3, or r4, respectively, apart from the center O2 on the effective section of the optical member L2, similar to FIG. 1A. In this case, since all the directions of the fast axis W21, W22, W23, W24 at the measuring points P21 to P24 are normal to the direction of the straight line Q2, i.e., to the radial direction, all the signed birefringence values A21, A22, A23, A24 at the measuring points P21 to P24 are expressed with the minus sign. When these signed birefringence values A21 to A24 at the measuring points P21 to P24 illustrated in FIG. 2A, obtained as described above, are plotted against the distance in the radial direction, the distribution obtained is, for example, the profile as illustrated in FIG. 2B.
FIG. 3A is a schematic diagram to show directions of the fast axis at birefringence measuring points P31, P32, P33, P34, and P35 the distance r1, r2, r3, r4, or r5, respectively, apart from the center O3 on the effective section of the optical member L3, similar to FIG. 1A. In this case, the directions of the fast axis W31, W32, W33, W34, and W35 at the measuring points P31 to P35 are such that those at the measuring points P31 to P33 are parallel to the direction of the straight line Q3, i.e., to the radial direction and those at the measuring points P34, P35 are perpendicular to the radial direction, and thus the distribution of the signed birefringence values A31 to A35 at the measuring points P31 to P35 against the distance in the radial direction is the profile as illustrated in FIG. 3B.
Next, the xe2x80x9csigned birefringence characteristic value of the entire projection optical systemxe2x80x9d in the present invention will be described below based on FIG. 4A and FIG. 4B.
FIG. 4A is a schematic side view in which m optical members constituting the projection optical system are arranged in order from the light source. FIG. 4B is a schematic, cross-sectional view to show the effective section normal to the optical axis, of the optical member Li located at the ith position from the light source out of the m optical members illustrated in FIG. 4A.
In the present invention, it is assumed that the distribution of birefringence values in the optical member is uniform in the direction of the thickness of the member parallel to the optical-axis direction but nonuniform in the radial direction on the effective section normal to the optical axis. Here the xe2x80x9ceffective sectionxe2x80x9d means an area under irradiation of light in the plane normal to the optical axis of the optical member. An intersection of the effective section with the optical axis is defined as a center of the effective section and the radius thereof as an effective radius of the effective section of the optical member. In the measurement of the signed birefringence characteristic value of the entire projection optical system, since the sizes of the effective sections are different among the optical members, the sizes of the effective sections of all the optical members are preliminarily normalized so that the maximum effective radius rn of each optical member becomes one as illustrated in FIG. 4A.
When a plurality of beams are radiated into the plane normal to the optical axis with the center at the intersection between the optical member and the optical axis, the sizes of the effective sections of all the optical members are preliminarily normalized so that the maximum effective radius rn of each optical member becomes one for each of the effective sections corresponding to the individual beams.
Further, in the cases wherein beams having the shape other than the circular cross section, for example, beams of the ring section or the elliptic section are radiated into the plane normal to the optical axis with the center at the intersection between the optical member and the optical axis, the sizes of the effective sections of all the optical members are also preliminarily normalized so that the maximum effective radius rn of each optical member becomes one for each of the effective sections corresponding to the individual beams.
For example, when the beams of the ring section are radiated, the sizes of the effective sections of all the optical members are preliminarily normalized so that the maximum outside radius of the ring becomes one, and the measurement of signed birefringence values can be performed in a manner similar to the measurement with the beams of the circular cross section described hereinafter. When the beams of the elliptic section are radiated, the sizes of the effective sections of all the optical members are preliminarily normalized so that the maximum outside length of the major axis of the ellipse becomes one, and the measurement of signed birefringence values can be carried out in a manner similar to the measurement with the beams of the circular section described below.
For measuring the signed birefringence characteristic value of the entire projection optical system, a first step is to establish a hypothetical model of concentric circles Cij with the center Oi and with their respective radii from the center on the effective section for one optical member Li, as illustrated in FIG. 4B. Then a birefringence value is measured at the kth measuring point Pijk on the jth concentric circle Cij with the radius of rj from the center Oi. Further, the sign is assigned to the measurement from the relation between the direction of the fast axis and the radial direction at the measuring point Pijk to obtain the signed birefringence value Aijk at the measuring point Pijk.
Here the character xe2x80x9cixe2x80x9d represents the numbers (i=1, 2, . . . , m; 2xe2x89xa6m) of the optical members L forming the projection optical system. Further, the character xe2x80x9cjxe2x80x9d represents the numbers (j=1, 2, . . . , n; 1xe2x89xa6n) of the concentric circles C with the center on the optical axis and with their respective radii from the optical axis different from each other, hypothetically given on the effective section normal to the optical axis in the optical member L. Further, the character xe2x80x9ckxe2x80x9d represents the numbers (k=1, 2, . . . , h; 1xe2x89xa6h) of measuring points on the circumference of the concentric circles C. In this way the signed birefringence values Aij1 to Aijh are measured at the predetermined measuring points Pij1 to Pijh on each single concentric circle cij.
Then an average signed birefringence value Bij, which is an arithmetic mean of the signed birefringence values at the measuring points on the circumference of the concentric circle Cij in the optical member Li, is calculated according to the equation below.                               B          ij                =                                            ∑                              k                =                1                            h                        ⁢                          A              ijk                                h                                    (        6        )            
Then Eij, which indicates an average signed birefringence amount as the product of the average signed birefringence value Bij and the apparent thickness Ti, is calculated according to the equation below.
Eij=Bijxc3x97Tixe2x80x83xe2x80x83(5)
In this equation Ti represents the apparent thickness of the optical member Li. This apparent thickness is either one properly selected from an average of thicknesses in the effective section of the optical member Li and an effective thickness based on matching with other members above and below the optical member Li when placed in the optical system.
Then an average change amount Gj of signed birefringence values, which is a result of division of the summation of average signed birefringence amounts Eij in the entire projection optical system by the total path length D, is calculated according to the equation below.                               G          j                =                                            ∑                              i                =                1                            m                        ⁢                          E              ij                                D                                    (        3        )            
In this equation D represents an apparent total path length of the entire projection optical system indicated by the following equation.                     D        =                              ∑                          i              =              1                        m                    ⁢                      T            i                                              (        4        )            
Then the signed birefringence characteristic value H of the entire projection optical system, which is a result of division of the summation of average change amounts Gj of the signed birefringence values in the entire projection optical system by the number n of concentric circles, is calculated according to the equation below.                     H        =                                            ∑                              j                =                1                            n                        ⁢                          G              j                                n                                    (        2        )            
When the signed birefringence characteristic value H of the entire projection optical system thus calculated satisfies the following equation, the entire projection optical system demonstrates excellent imaging performance and the projection exposure apparatus provided with such a projection optical system shows excellent resolution.
xe2x88x920.5xe2x89xa6Hxe2x89xa6+0.5 nm/cmxe2x80x83xe2x80x83(1)
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.